As is generally well known in the art of controlling machinery, fluid filled conduits for control of machinery have been in widespread use for more than a century. Railroad braking systems based on a compressed air line as developed from the classical Westinghouse airbrake system are an example. Other examples may be found in the control of highway vehicles, submarines, aircraft, etc. The Westinghouse airbrake is a particularly interesting example, since it has a fail-safe feature in that failure of the brake air line resulting in a loss of brake line pressure causes application of brakes throughout the entire train.
Generally these systems have some amount of time delay between the initiation of a control signal and the actuation of the device being controlled. These delays can be quite significant for large systems. In particular, in a railway braking system, the time needed for a pressure decrement to travel along the length of a brake air line from a locomotive to a remote part of the train consist may be as long as a minute or longer.
More rapid methods of signal transmission, for example, by electrical wires, electromagnetic signals, or optical transmission, are known.
For railroad braking systems, the classical brake air line may be combined with radio transmission, particularly in a train having locomotives distributed at various locations along the train.
The WABCO Epic .RTM. brake system combined with a radio communication link from Harris Locotrol.RTM. provides a system in which a brake application signalled by the lead locomotive of a train is accompanied by a radio signal sent from the lead locomotive to slave locomotives in portions of the train remote from the lead locomotive. As usual, with railroad airbrake systems, the lead locomotive dumps brakeline air, which sends a pressure decrement down the line of cars, causing a brake application as it proceeds. The radio signal is immediately received in locomotives remote from the lead locomotive, and these also begin venting brakeline air. Brakeline pressure decrements then begin to travel along the succession of railroad vehicles from each slave locomotive, causing the brakes to be applied as the pressure decrement reaches each vehicle.
Operation of this system requires, in each locomotive which supplies air to the brakeline, a measurement of the flowrate of air from a main air reservoir in the locomotive to the brakeline of the locomotive. The air pressure in the main air reservoir is maintained by a compressor in the locomotive. This flowrate can be used for a number of purposes. One thing it is used for is to determine the leakage flowrate. This is the flowrate of air which leaks out of the brakeline anywhere in the train. This flowrate is also used when the train is being prepared for travel, or after a brake application. In both of these cases, the brakeline pressure must be brought up to the operating pressure value. By measuring the flowrate to the brakeline, the system can determine when the brakeline is charged. This occurs when the measured flow through the orifice is approximately equal to the leakage flowrate.
The flowrate of air is measured by an orifice, such as the air path constriction in the L19 flowblock, which is located between the main air reservoir, and the brakeline. A pressure is obtained either from a pressure tap in the reservoir, or at a pressure tap at a point in the air path upstream of the orifice, and another pressure is obtained at a pressure tap at a point in the air path downstream of the orifice. These pressures are then used to determine the flowrate through the orifice. The flowrate is calculated by a formula, discussed in detail hereinafter, which includes the square root of the pressure drop across the orifice. Two types of transducer configurations are commonly used. In one configuration, two pressure sensing transducers are used. One measures a pressure upstream of the orifice, and one measures a pressure downstream of the orifice. The values of these pressures are subtracted from each other to obtain the differential pressure.
A preferred and more accurate method is to use a differential pressure transducer, which directly measures the pressure differential across the orifice. This transducer, for example, may have a diaphragm which has a space on one side which is connected to a pressure port upstream of the orifice, and a space on the other side which is connected to a pressure port downstream of the orifice.
The formula used in the prior art, which is based on the square root of the pressure difference, has an accuracy in the range of 10%. For some of the older systems, this has been adequate, but for systems employing remote locomotives controlled by a radio link, this accuracy is not sufficient.
Another difficulty with the prior art systems is that in normal operations, when the brakes are not applied, and the brakepipe is fully charged, the brakeline continuously leaks air, and demands air from the main reservoir. Hence, the main reservoir loses pressure continuously through the orifice supplying the brakeline, and is resupplied with air by pulses of air originating in the compressor. A brake control valve placed downstream of the orifice, and upstream of the brakepipe, controls the pressure downstream of the valve. Because the flow necessary to supply the leakage of air from the brakeline remains substantially constant, the brake control valve compensates for changes in main reservoir pressure to maintain a constant flow through the orifice. The prior art equation, which calculates the flow as the square root of the differential pressure across the orifice then incorrectly indicates a change in the flow rate. This erroneous indication triggers alarms which are sensitive to changes in flow which would be caused by a sudden major leak in the brakepipe.